linux-kernel - Re: [PATCH v2 2.6.38-rc8-tip 4/20] 4: uprobes: Adding and remove a uprobe in a rb tree.

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Date:	Wed, 16 Mar 2011 08:54:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	SystemTap <systemtap@...rces.redhat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 2.6.38-rc8-tip 4/20] 4: uprobes: Adding and remove a
 uprobe in a rb tree.

On Tue, 2011-03-15 at 23:42 +0100, Eric Dumazet wrote:
> Le mardi 15 mars 2011 à 20:48 +0100, Peter Zijlstra a écrit :
> > On Tue, 2011-03-15 at 20:22 +0100, Thomas Gleixner wrote:
> > > I am not sure if its a good idea to walk the tree
> > > > as and when the tree is changing either because of a insertion or
> > > > deletion of a probe.
> > > 
> > > I know that you cannot walk the tree lockless except you would use
> > > some rcu based container for your probes. 
> > 
> > You can in fact combine a seqlock, rb-trees and RCU to do lockless
> > walks.
> > 
> >   https://lkml.org/lkml/2010/10/20/160
> > 
> > and
> > 
> >   https://lkml.org/lkml/2010/10/20/437
> > 
> > But doing that would be an optimization best done once we get all this
> > working nicely.
> > 
> 
> We have such schem in net/ipv4/inetpeer.c function inet_getpeer() (using
> a seqlock on latest net-next-2.6 tree), but we added a counter to make
> sure a reader could not enter an infinite loop while traversing tree

Right, Linus suggested a single lockless iteration, but a limited count
works too.

> (AVL tree in inetpeer case).

Ooh, there's an AVL implementation in the kernel? I have to ask, why not
use the RB-tree? (I know AVL has a slightly stronger balancing condition
which reduces the max depth from 2*log(n) to 1+log(n)).

Also, if it does make sense to have both and AVL and RB implementation,
does it then also make sense to lift the AVL thing to generic code into
lib/ ?

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